Recently, a motor control apparatus has been put to practical use that includes a speed estimator that estimates the rotational speed of a motor from an induced voltage generated by the rotation of the motor and perform sensorless control. In this type of the motor control apparatus, there is known a motor control apparatus that performs maximum torque control in which the reluctance torque is effectively used. The maximum torque control is performed by using a speed estimated error generated by intentionally inducing an error in the true value of a q-axis inductance, which is used by the speed estimator for calculation, and causing the control axis to match the maximum torque operating point.
First and second methods are disclosed in Japanese Patent Application Laid-open No. 2009-291072 as a method of shifting the q-axis inductance, which is a parameter for calculation in the speed estimator, from its true value. The first method is a method that uses a parameter L for calculation, which is set to a value satisfying Ld≦L<Lq. The second method is a method that introduces a dm-qm coordinate system, in which the rotational axis, whose direction matches the direction of the current vector that realizes the maximum torque control, is the qm axis and the rotational axis orthogonal to the qm axis is the dm axis, and that uses a parameter Lm for calculation. Motor parameters Ld, Lq, and Φa are used for calculating the parameter Lm for calculation.
However, the first method has a problem in that because the parameter L for calculation is a fixed value, the accuracy of the maximum torque control is reduced as the load increases.
In the second method, the parameter Lm for calculation is a function of the q-axis inductance; therefore, the second method can improve the first method. However, because the motor parameters (Ld, Lq, and Φa) are used for calculating the parameter Lm for calculation, if there is an error in the initial setting value or the motor parameters change due to temperature or load, an error occurs in the parameter Lm for calculation in accordance with the error in the motor parameters. Therefore, there is a problem in that the maximum torque control cannot be obtained and moreover the speed estimator becomes unstable.